FIG. 1 of EP B1 079 726 shows the main elements of the described measuring cell. The cell cavity is divided into two compartments by a mesh. As illustrated the mesh is horizontal, providing upper and lower compartments. A respective measuring electrode projects into each compartment. The operation of the cell in brief is that the paper stock is fed to the lower compartment to build a pad of fibrous material on the underside of the mesh, the cell being generally full of the water component of the stock. The pad is then subject to controlled pressure conditions across the pad and a potential difference measurement is made between the electrodes at the established pressures to obtain a measurement of streaming potential. This is discussed in detail in the prior patent.
The described measurement cell and its manner of operation are considered to provide a substantial advance in the art of measuring streaming potential, particularly in providing an effective means of on-line measurement on a paper-making machine. The cell particularly described in the patent utilises stainless steel electrodes. It was found that in long-term use, irregularities and drifts could arise in measurements made with the cell. Investigation has revealed that these irregularities were related to an electrochemical potential effects arising from oxidation of the stainless steel. The nature of the surface oxide film that forms on stainless steel is very variable.
In seeking for alternative materials it has also been found that avoiding the build up of an oxide film is not necessarily the answer. The present investigation has shown that there may also be a capacitance effect at the electrode/water interface which causes drift in potential measurement. This capacitance is very high. Measurements on gold electrodes, which do not readily oxidize, have indicated a capacitance value of 20 .mu.F/cm.sup.2.
This capacitance is a direct consequence of polarization of the electrode in the stock being measured. There is an inevitable but quantifiable polarization in tap water. However, polarization effects also result from chemicals being absorbed onto the electrodes from the stock being measured. These are not quantifiable, particularly having regard to the wide variation that occurs in the constituents of paper-stock. It has been found that gold electrodes suffer from such absorption. The electrode then forms the equivalent of a polarized electrolytic capacitor with an ionised layer in the stock within a few molecules of the electrode surface. This gives rise to the high capacitance per unit area quoted above and the better the quality of the capacitor formed, that is the lower the leakage current, the less suited is the electrode for the measurements required.
Thus there is a general requirement to use for the electrodes metals that have the least possible polarization and for any resulting capacitance to be of poor quality, that is have a high leakage current. The metals proposed in accordance with the present invention have a reversible electrochemical reaction with pure water to oxidise the metal.